1. Field of the Invention
This invention relates to a voltage control device for controlling a voltage used for driving, for example, a motor operated with a high internal impedance battery employed as a power source.
2. Description of the Related Art
Generally, the devices of the above-stated kind have been arranged to operate in one of two different modes, one mode when the motor is rotated and in another mode when the motor is in repose. Some of them are arranged to have the motor driven at a high voltage when the motor is rotating and to have a weak current continuously applied at such a low voltage that never causes any phase deviation of the motor when the motor is in repose.
In cases where a battery is used as a power source, the method of continuously applying a weak current when the motor is in repose has not been adopted for the purpose of saving the electrical energy of the battery from being wasted. In this case, the above-stated method is replaced with a method, wherein, before rotating the motor, a phase at which the motor is brought to a stop at the end of a previous operation is excited for a given period of time T0, as shown in FIG. 8 of the accompanying drawings, in such a way as to adjust the phase of control to the actual phase of the motor. This method prevents the motor from going out of control due to any slight phase deviation that occurs when the motor is previously brought to a stop. An example of this method has been disclosed, as a stepping motor control method, for example, in Japanese Lid-Open Patent Application NO. SHO 59-201885. Further, in a case where the motor is to be driven at a low voltage by means of a battery, the battery consumption can be lessened by preventing the current from being wasted in the following manner. In some cases, not much current is required in obtaining a required amount of torque, while a relatively greater amount of current is required in other cases. Therefore, the battery consumption can be lessened by varying the required amount of driving voltage stepwise, to several values. In FIG. 8, a reference symbol .DELTA.VB3 denotes a fluctuating range of the power supply voltage. In such a case, as shown in FIG. 8, a stop-phase exciting action for phase adjustment is carried out after the driving voltage is changed to a value between a voltage VM0 and VM1. (In the case of FIG. 8, a rush current preventing capacitor which will be described later herein is not inserted.)
However, in driving the motor after changing the driving voltage from a low voltage VM0 to a high voltage VM2 as shown in FIG. 8, the conventional device presents the following problems.
(1) With the driving voltage values (VM0 to VM2) arranged stepwise, the battery voltage drops when the motor phase adjustment is performed at a high voltage. As a result, a conversion efficiency of a voltage converter which converts the battery voltage to the motor driving voltage or that of some other voltage converter disposed within the same apparatus might be lowered. This eventually shortens the life of the battery.
(2) If the motor is caused to rotate immediately after adjustment of the stop phase thereof, a large power supply is required from the battery because of a large load. In that case, as shown in FIG. 8, the battery voltage VB is dropped by the phase adjustment and also gradually drops after the start of the motor rotation. Therefore, this brings about the same result as in the case of the problem (1) above.